The present invention relates to a laser interferometer type length measuring apparatus and a positioning method which can provide improved measuring accuracy, and more particularly to an interferometer type length measuring apparatus and a method which are preferably used to improve the accuracy and stability in positioning a wafer in a semiconductor production system.
One of conventional laser interferometer type length measuring apparatus with high measurement accuracy is disclosed in U.S. Pat. No. 4,984,891 assigned to the same applicant of the present invention. This prior art intends to compensate an air fluctuation in the measurement by the following manner. As shown in FIG. 6, a laser beam 20 for compensation is independently located in the neighborhood of a laser beam 4 for measurement. A reflected laser beam of the light beam 20 for compensation reflected from a mirror 26 apart from an interferometer 19 by a predetermined distance interfere with a reference laser beam in the interferometer 19. The interference light 23 thus obtained is processed to provide an output 30 indicative of an air fluctuation. On the other hand, a reflected laser beam of the laser beam 4 for measurement reflected from a mirror 5 interfere with the reference laser beam in another interferometer 3. An interference light 7 thus obtained is processed to provide another output 29 indicative of a displacement of an object (i.e. a displacement of an object 6 provided with the mirror 5) including the air fluctuation. Both outputs thus obtained are taken in a computing device 31 to provide an output 32 indicative of the displacement free from the air fluctuation.
However, the above prior art has the following disadvantage. If there are very non-uniform distributions of temperature and air pressure on the laser beams, and if the laser beams for measurement and for compensation are in different environments, the compensation will be incomplete so that some measurement errors will occur.
More specifically, a difference will occur between the temperature distributions on both beams because of an air fluctuation due to heat from a laser tube, and also a large distribution of the air pressure will occur only on the laser beam for measurement during a measuring object moves. Such phenomena are actually observed in conventional apparatus.